simple solution, especially if you live in rented property, buy the new jbl room correction monitors.
Stunning, and far better than faffing about with foam, unless you are an accoustic technician.
There is some great new room correction software out now, made by M-Audio I think??
I`ll find the link, but it works on the same principle as the JBLs, but is a lot cheaper.
Basically it analyses your room, and applies filters prior to your sound card output to correct for standing waves etc.

Other than that, bass traps in the corners (verticle and horizontal, anti-scatter/reflection tiles, to either side of the listening spot, and if the wall behind is close, just behind as well.
Possibly some deflectors directly above the monitoring position as well, to essentially stop any symmetrical modes being present.

All this can set you back a fair wack, more than 250.

I think the new room mode correction software costs about 250 AND it moves wherever your studio moves.

I plumped for the monitors, and man, I`m so happy I nearly cry every time I switch them on.
My mastering has improved so much since I got em